Shunt active power filters are used to minimize the harmonic disturbances created by non-linear loads as they improve the filtering efficiency, and also solve many issues arising with classical passive filters. During the design of such a system, special attention has to be paid to the load currents, which are intended to be compensated. Together with the modulation strategy employed, the load currents are the variables that determine the circulating current in the power devices of the selected Voltage Source Converter (VSC). Especially in 3-level NPC (Neutral Point Clamped) active filters, the commonly irregular load can lead to an uneven loss distribution in the semiconductors of a bridge leg. As in every converter, the losses in the most stressed device limit the switching frequency and the power capability, a de-rating of the converter current can become mandatory to ensure long term-stability of the system.
The semiconductor chips assembled in a standard commercial 3-level NPC bridge leg module are mostly dimensioned and rated neglecting the loss distribution over the specific elements. In this manner, due to the issue of loss distribution, the usage of these devices often results in an oversized design with expensive and weakly utilized semiconductor area. Additionally, modulation schemes used to enhance the system efficiency can contribute even more to the uneven loss distribution, increasing the difference of the operating temperature of the transistors and diodes inside the power module and/or widening their thermal cycling. The thermal mismatch of components leads to induced thermal stresses on the materials within the module and thermo-mechanical damage could arise. Consequently, the design of 3-level NPC active filters becomes rather complex as the desired characteristics of high power density, efficiency and component reliability could work against each other.
Due to the unequal distribution of losses and consequent mismatch of junction temperature distribution among the bridge-leg's semiconductors, in the situation of high power converters, for example, the usage of NPC power modules can result in low semiconductor utilization. In this way, the usage of single semiconductor devices, rated differently, are more indicated to build the bridge-legs of the converter. NPC systems, employing single semiconductors similarly rated, usually have these devices installed in separate heat sinks, in order to achieve a good thermal decoupling of the individual components. However, the usage of different single semiconductors and/or separate heating sinks can result in cost increments and bulky systems.